Method for regenerating a catalyst with a sulphurus active phase and contaminated by a mercury compound

ABSTRACT

The invention features a method for treating a supported catalyst with a sulphur compound based active phase and a mercury compound contaminated mineral support. This method consists in implementing the following steps: the said catalyst is contacted with a basic aqueous solution of at least one sulphide selected among alkaline metal sulphides, alkaline-earth metal sulphides or ammonium sulphide; then the catalyst is separated from the basic aqueous solution. At the end of this process, the treated catalyst shows the same characteristics as before its contamination by mercury.

The present invention relates to a process for treating spent catalystscontaminated by mercury so as to regenerate them and reuse them.

At the present time, the treatment of contaminated, spent products isbecoming an activity of increasing importance. Environmental constraintsare requiring all types of liquids, solids or gases to be purified forthe purpose of lowering their content of pollutants and of avoiding thelatter being discharged into the environment.

Many catalysts have been developed for the purpose of eliminating theseimpurities by absorption, adsorption or chemical reactions. However,once the contaminated products have been purified, the impurities havehigh concentrations in these catalysts, which must be stored ordestroyed.

Currently, the trend is to try to treat these spent catalysts so as toremove the impurities therefrom. The aim of this treatment is, on theone hand, to recover impurities for the purpose of their reutilizationor conversion into an easily storable form and, on the other hand, toregenerate the spent catalysts so as to be able to reuse them. Thissecond aspect furthermore allows savings to be made since a new chargeof fresh catalyst does not have to be purchased.

Among impurities particularly harmful to the environment, mercury, inany of its forms, is of great importance. In particular, it is presentin natural gas or petroleum cuts. One way of removing the mercury fromthese gases is to bring them into contact with a catalyst based on asulphur compound, for example a supported catalyst having a mineralsupport and an active phase based on a sulphur compound, for examplecopper sulphide.

Mercury present in the gases reacts on contact with these catalysts toform a mercury compound, especially mercury sulphide, which remainsadsorbed or absorbed on the catalyst.

Currently, spent catalysts comprising these mercury compounds aredestroyed or stored; and a new charge of fresh catalyst must be used inorder to continue treating the gases.

One object of the present invention is to provide a process for treatingspent catalysts of this type which make it possible not only to removethe mercury therefrom but also to regenerate the initial active phase ofthe said catalyst so as to be able to use it again in the sameapplication without having to redeposit a new active phase.

To this end, the invention relates to a process for regenerating asupported catalyst having an active phase based on a sulphur compoundand a mineral support, contaminated by a mercury compound, in which thefollowing steps are carried out:

the said catalyst is brought into contact with a basic aqueous solutionof at least one sulphide chosen from alkali metal sulphides,alkaline-earth metal sulphides or ammonium sulphide,

then the catalyst is separated from the basic aqueous solution.

Further advantages of the invention will appear more clearly on readingthe description and the examples.

The invention therefore relates to a process for regenerating asupported catalyst having an active phase based on a sulphur compoundand a mineral support, contaminated by a mercury compound in which thefollowing steps are carried out:

the said catalyst is brought into contact with a basic aqueous solutionof at least one sulphide chosen from alkali metal sulphides,alkaline-earth metal sulphides or ammonium sulphide,

then the catalyst is separated from the basic aqueous solution.

According to the process of the invention, the spent catalyst from whichit is necessary to remove the mercury compounds is brought into contactwith a basic aqueous solution of one or more sulphides, as definedabove.

Preferably, the basic aqueous solution is a solution of sodium sulphideor potassium sulphide.

According to a preferred way of carrying out the process of theinvention, this basic aqueous solution is obtained by mixing an aqueoussolution of one of these sulphides and sodium hydroxide.

In this case, the concentration of the sodium hydroxide is preferably atleast 0.1M in the mixture, especially of at least 0.5M.

In general, it is recommended to use a basic aqueous solution whose pHis at least 10, preferably at least 12.

In order to optimize the quantities of products to be employed, thecatalyst is preferably brought into contact with the basic aqueoussolution in quantities such that the molar ratio of the sulphur providedby the basic aqueous solution to the mercury present in the catalyst(S/Hg) is at least 5, preferably at least 10.

Likewise, it is recommended to bring the catalyst into contact with thebasic aqueous solution in quantities such that the ratio of the volumeof the basic aqueous solution to the mass of the catalyst (L/S) is atleast 2 l/kg, preferably at least 5.

The contact time between the spent catalyst and the basic aqueoussolution may vary. In general, it is between 30 minutes and 8 hours. Acontact time of approximately 2 hours is usually sufficient.

According to a preferred method, the treatment is carried out under thefollowing conditions:

the sulphide used is sodium sulphide,

the basic aqueous solution of sodium sulphide has an Na₂ S concentrationof 0.5M,

the basic aqueous solution of sodium sulphide has an NaOH concentrationof 0.5M,

and the L/S ratio is 5 l/kg.

The solid may be separated from the treatment solution by varioustechniques, such as filtration, draining, settling, flocculation and anyother technique known to those skilled in the art.

Normally, after the catalyst has been separated from the basic aqueoussolution, the catalyst may be washed using an aqueous washing solution.

This aqueous washing solution may be a fresh basic aqueous solution of asulphide (or sulphides), as defined above, or simply water.

In general, after the catalyst has been separated from the basic aqueoussolution and optionally after the catalyst has been washed, the catalystmay be dried.

This drying operation may be carried out by an oven treatment, thetreatment being with a purge of a gas which is inert in terms oftemperature, at a temperature of between 50 and 100° C.

After the catalyst has been separated from the basic aqueous treatmentsolution, it may be advantageous to reutilize the said basic aqueoussolution by treating it in the presence of an acid compound. Byintroducing an acid compound into the basic aqueous solution coming fromthe process according to the invention, the mercury compound being in asoluble form therein, is precipitated so that it can be easily recoveredby a simple filtration operation.

Next, the solution separated from the precipitated mercury compound istreated in order to recover H₂ S, optionally for the purpose ofrecycling it. Thus, the sulphides contained in this solution aredecomposed, for example by adding an acid compound, in order to give H₂S which is removed by volatilization (heating, sparging with gases suchas air, N₂, . . . ) and then recovered by absorption in a basicsolutions, for example based on sodium hydroxide. This basic solutioncoming from the H₂ S absorption may be reutilized for a new cycle fortreating a charge of spent catalyst.

The acid compounds allowing the mercury to be precipitated and thesulphides to be decomposed may be chosen from hydrochloric acid orsulphuric acid. It is preferable to add an acid compound to the basicaqueous solution so that the pH of the mixture is at most 7, preferablyat most 2.

The process according to the invention is particularly useful fortreating a supported catalyst having an active phase based on coppersulphide. The quantity of this compound in the catalyst to be treatedmay vary--it may be as much as 6% by weight with respect to the mass ofthe catalyst.

The supported catalyst may have a mineral support based on alumina,silica, silica-alumina or carbon.

Removal of the mercury is followed by analysis by means of various knownanalytical techniques for assaying mercury, especially by plasmaemission spectrometry.

The advantage of the process according to the invention is that not onlyis mercury removed from the catalyst but also that the active phase ofthe catalyst is regenerated and the support has the same characteristicsas during the first time it was used.

The mercury sulphide may be reutilized.

The following examples illustrate the invention without, however,limiting the scope thereof.

EXAMPLE

The starting point is a catalyst having the following characteristics:

Support:

alumina

SSA=125 m² /g

TPV=0.75 cm³ /g

Active phase:

CuS

amount: 15.3% by weight with respect to the catalyst.

This catalyst is used in a unit for removing mercury from a natural gas.At the end of the gas treatment, the catalyst comprises 5.2% by weightof mercuric sulphide.

The catalyst is extracted from the unit in order to be treated accordingto the process of the invention.

1 kg of spent catalyst is introduced into a reactor. 5 liters of anaqueous solution of sodium sulphide and sodium hydroxide, having thefollowing characteristics:

pH=13-14

NaOH concentration=0.5M

Na₂ S concentration=0.5M

S/Hg molar ratio=11.2 are prepared.

The treatment solution is made to flow over the catalyst continuouslyfor 2 hours.

The sodium sulphide solution is then drained off from the reactor. Thecatalyst is washed with another sodium sulphide solution having the samecharacteristics as the previous solution, and then washed with water.

Finally, it is dried.

The residual mercuric sulphide content is less than 0.1% by weight withrespect to the catalyst. Next, hydrochloric acid is added to thetreatment solution, which contains mercuric sulphide, so as to reach apH of less than or equal to 2. Under these conditions, mercuric sulphideprecipitates. Its residual concentration in the solution after thehydrochloric acid has been added is less than or equal to 0.02 mg/l.

What is claimed is:
 1. Process for regenerating a supported catalysthaving an active phase based on a sulphur compound and a mineralsupport, contaminated by a mercury compound, comprising the followingsteps:bringing said catalyst into contact with a basic aqueous solutionof at least one sulphide comprising alkali metal sulphides,alkaline-earth metal sulphides or ammonium sulphide; and separating thecatalyst from the basic aqueous solution.
 2. Process according to claim1, wherein the basic aqueous solution is a solution of sodium sulphideor potassium sulphide.
 3. Process according to claim 1, wherein thebasic aqueous solution is obtained by mixing an aqueous solution of atleast one sulphide, comprising alkali metal sulphides, alkaline-earthmetal sulphides or ammonium sulphide, with sodium hydroxide.
 4. Processaccording to claim 3, wherein the concentration of the sodium hydroxideis at least 0.1M.
 5. Process according to claim 1, wherein the pH of thebasic aqueous solution is at least
 10. 6. Process according to claim 1,wherein the catalyst is brought into contact with the basic aqueoussolution in quantities such that the molar ratio of the sulphur providedby the basic aqueous solution to the mercury present in the catalyst(S/Hg) is at least
 5. 7. Process according to claim 1, wherein thecatalyst is brought into contact with the basic aqueous solution inquantities such that the ratio of the volume of the basic aqueoussolution to the mass of the catalyst (L/S, in l/kg) is at least
 2. 8.Process according to claim 1, wherein, after the catalyst has beenseparated from the basic aqueous solution, the catalyst is washed usingan aqueous washing solution.
 9. Process according to claim 8, wherein,after the catalyst has been separated from the basic aqueous solutionand after the catalyst has been washed, the catalyst is dried. 10.Process according to claim 1, wherein, after the catalyst has beenseparated from the basic aqueous solution, said basic aqueous solutionis reutilized by adding an acid compound.
 11. Process according to claim10, wherein an acid compound is added to the basic aqueous solution sothat the pH of the mixture is at most
 7. 12. Process according to claim1, wherein the supported catalyst has an active phase based on coppersulphide.
 13. Process according to claim 1, wherein the supportedcatalyst has a mineral support based on alumina, silica, silica-aluminaor carbon.
 14. Process according to claim 1, wherein the mercurycompound is mercury sulphide.
 15. Process according to claim 5, whereinthe pH of the basic aqueous solution is at least
 12. 16. Processaccording to claim 7, wherein the catalyst is brought into contact withthe basic aqueous solution in quantities such that the ratio of thevolume of the basic aqueous solution to the mass of the catalyst (L/S,in l/kg) is at least 5.